Ion vacuum gauge



Jan. 9, 1951 s. M. M NEILLE ION VACUUM GAUGE Filed July'27, 1944 noun)INVENTOR. JZe ohen m. MzZcA ezZZZe FfIE.B

am/ r- Patented Jan. 9, 1951 ION VACUUM GAUGE Stephen M. MacNeille, OakRidge, Tenn., as-

signor, by mesne assignments, to the United States of America asrepresented by the United States Atomic Energy Commission ApplicationJuly 27, 1944, Serial No. 546,902

7 Claims. 1

This invention relates to gas pressure measurement and particularly to amethod and apparatus for ascertaining the pressure of a gas by notingthe relationship between electrons bombarding the gas and the ionizationresulting therefrom.

It has long been known that electrons in passing through a gas at lowpressure will ionize the gas, and that the number of ions formed is afunction of the gas pressure. In the usual arrangement the intensity ofthe electronic bombardment has been held constant and the pressureascertained by noting the intensity of the ionization produced thereby.

These devices, designated ionization gauges, comprise a source ofelectrons, usually a heated filament, and two spaced electrodes, one ofwhich is positive and the other negative with respect to the filament orcathode. The positive electrode. the anode, collects the electroncurrent and the negative electrode, the plate, collects the positiveions. The practice in the past has been to control the filament emissionto maintain a constant electron current to the anode and measure theplate current as an indication of the amount of ionization and hence thepressure.

In accordance with the present invention, an ionization gauge isoperated with its ion current held constant at a predetermined value bysuitable regulation of the electronic discharge which preferably isaccomplished by varying the temperature of the filament.

An object of the present invention is the provision of a circuitarrangement which will operate to maintain constant the ion current inan ionization gauge and which will measure the electron current in termsof gas pressure.

Other objects and advantages of the invention will become apparent fromthe following description when taken in connection with the accompanyingdrawing in which Fig. l is a. simplified circuit diagram showing theoperating principle of the invention; and,

Fig. 2 is a circuit diagram illustrating a preferred form of theinvention.

As shown in Fig. 1 an ionization tube II) has a 2 cult constants aresuch that when a predetermined current, for example one microampere,flows to the plate [5 as indicated by a suitable meter [7, the electroncurrent is a known function of the pressure. A suitable meter l8responds to this electron current and may be calibrated in pressureunits if desired. The source I2 is variable so that when the pressurechanges, the temperature of the cathode II can be adjusted to keep theion current at its predetermined value. Any change in filament orcathode current is reflected in the electron current, and thus thepressure change appears in the reading of the meter It.

It will be appreciated that the design of the control arrangement willbe such that at or near the pressures expected to be measured, as, forinstance, 10- mm. of mercury, the filament current required will be lowenough to insure long life. This aspect is important and involves one ofthe most advantageous features of the invention. This will be clear ifit isremembered that when the electron current is held constant, asheretofore, an increase in pressure requires an increase in filamentcurrent whereas, with the constant ion current arrangement of thepresent invention, an increase in pressure requires less filamentcurrent. of course, in the unlikely event that the pressure decreasedconsiderably beyond the expected pressure, the filament current wouldhave to be increased in order to keep the ion current constant. However,unexpected increases in the degree of vacuum seldom occur whereasincreases in pressure, especially in large volume systems, occur withsuch frequency as to be almost expected.

A self regulating arrangement incorporating the invention is shown inFig. 2 as comprising an ionization gauge tube 20 having its filament 2|connected to be energized from a suitable source of alternating current22 through a saturable reactor 23 and a transformer 24. As an aid inexplaining the circuit arrangement, certain numerical values will begiven which have been found to be satisfactory. It will, however, beunder-' stood that that circuits with other values may readily bedevised. Also connected to the source of supply 22 is a conventionalpower supply including a full wave rectifier 25 the output of which isfiltered and impressed across a voltage divider comprising theresistances 26, 21, and 28 having associated therewith two voltageregulating tubes 29 and 3|! and a resistance 3|. The filament 2! of theion tube 20 is connected between resistances 26 and 21 so that it willbe held approximately 15 volts above ground potential. The plate 32 ofthe tube 20 is connected to ground through a one megohm resistor 33thereby giving the plate 32 a potential of approximately volts negativewith respect to the filament 2|. The grid 34 of the ion tube isconnected between the two voltage regulating tubes 29 and 30 so that thegrid 34 is given a potential of approximately 150 volts above ground andabout 135 volts positive with respect to the filament 2|. With thesevoltages, it is evident that the grid 34 is an anode and will collectall the electrons emitted by the filament 2 I while the plate 32 beingthe most negative electrode will collect the positive ions which may beformed by electron bombardment.

The plate or ion current in flowing to ground through the resistor 33 isadapted to give 9. voltage drop of one volt and this drop is applied tothe grid 35 of a triode 35, which may be type 6SF5, the plate 38 ofwhich is connected through a resistance 31 to a 300 volt supply. Thepotential of plate 38 is applied through a resistance 39 to the controlgrid 40 of a second tube 4|, which may be type 6SH7, the cathode 42 ofwhich is held at 150 volts by proper connection to the voltage supply.The plate 43 of the tube 4| is connected through a D. C. winding 44 0fthe saturable reactor 23 to the 450 volt side of the power supply. Asstated above, the constants of the particular circuit being describedare such that regulation should take place with the ion currentcollected by plate 32 at a value of one microampere. This current inflowing through the resistance 33 applies a potential of one volt to thegrid 35 of the tube 36. In order to check the value of this current, thegrid 35 is adapted to be disconnected from this voltage and connected toa regulated potential of one volt by means of a push button switch 45 asshown. If pressing the switch 45 does not alter the meter reading (to belater described), it is established that the known predetermined platecurrent is flowing. This is the plate current which in accordance withthe invention is held constant. A milliammeter 46 is connected in theanode circuit of the ion tube 20 for measuring the magnitude of theelectron current necessary to maintain constant the plate currentflowing through the resistance 33. The electron current flowing throughthe meter 46 is a function of the gas pressure in the ion tube 20 andaccordingly the reading of the meter 46 is readily convertible into apressure reading. In order that this meter 46 may be used for difierentranges, a. series of shunts 41 are made selectively available by meansof a switch 48.

It will be noted that the tube 4| has in its screen grid lead a switch49 which when in its normal position as shown connects this screen gridto a voltage intermediate the voltages of the cathode 42 and the plate43 of the tube 4|. It has been found that when the filament 2| of thegauge tube 20 ages considerably, there is a tendency to oscillationwhich sometimes causes sparking across the D. C. winding 44 of thereactor 23. When this occurs, the switch 49 can be thrown to the rightto connect the tube 4| as a triode. This reduces somewhat thesensitivity of the circuit but it will still function.

The regulatin circuit just described operates in the following manner:As long as the pressure of the gas in the ion tube 20 remains constantand the plate current flowing through the resistance 33 remainsconstant, the meter 46 gives a reading indicating this pressure. If thepressure within the tube 20 starts to increase more ions will be formedand the current through the resistance 33 will tend to increase. Thiswill drive the grid 35 of the tube 36 more positive thereby increasingthe current flow through its plate 38 which thus becomes less positivethereby rendering the grid 40 of the tube 4| more negative whichdecreases the current to its plate 43. This decrease in plate currentthrough the tube 4| decreases the current through the D. C. winding 44of the saturable reactor 23 thereby increasing its impedance and thealternating current drop across it. This decreases the voltage to thetransformer 24; less voltage is applied to the filament 2| of the iontube 20; the filament 2| cools slightly and fewer electrons are emitted;less ionization occurs within the ion tube 20 and the ion currentremains constant. Although the ion current remained constant, theelectron current became less and the reading of the meter 46correspondingly changed to indicate the new pressure condition existingwithin the ion tube 20.

It is to be understood that the invention is not restricted to thespecific embodiments shown and described, since these were chosen forthe purpose of illustrating the invention, and many modifications willreadily suggest themselves to those skilled in the art. Other circuitarrangements may be employed for practising the invention, the scope ofwhich ispointed out in the appended claims.

I claim:

1. The method of detecting variations in pressure of a gas at pressuresless than one millimeter of mercury which comprises passing an electroncurrent through the gas to form positive ions, collecting said ions as acurrent, varying the intensity of the electron current to keep the ioncurrent constant at a predetermined value, and measuring the variationsin the electron current.

2. The method of measuring gas pressures less than a millimeter ofmercury, which comprises ionizing a portion of the gas at apredetermined rate which is independent of the pressure and measuringthe magnitude of the ionizing agent required to produce ionization atsaid predetermined rate.

3. In an ionization pressure gauge, an ionization tube comprising afilament, an anode and a plate immersed in a gas the pressure of whichis to be measured, voltage means for rendering the filament positivewith respect to the plate and negative with respect the anode, means forsupplying heating current to the filament to cause it to emit electrons,whereby electrons move to the anode and ions are formed in the gas andmove to the plate to form a current and means responsive to this ioncurrent for varying the heating current to the filament whenever the ioncurrent differs from a predetermined value.

4. In an ionization vacuum gauge comprising ionizing means, an ioncollector and an electron collector immersed in a gas the pressure ofwhich is to be measured, means for regulating the ion current comprisingmeans for deriving a potential from the ion current flow and meansresponsive to said potential controlling the electron current tomaintain said potential at a predetermined value.

5. In an ionization pressure gauge comprising an ionization tube havingan emitter and an ion collector immersed in a gas the pressure of whichis to be measured, in combination, means for regulating the emittercomprising means responsive to ionization in the tube, means comprisinga saturable reactor for controlling the power supplied to the emitterand said ionization responsive means being connected to control thesaturable reactor whereby to maintain the rate of ionization in the tubeat a predetermined value by regulating the emitter.

6. In an ionization pressure gauge comprising ionizing means and anelectron collector immersed in the gas the pressure of which is to bemeasured, in combination, means responsive to ionization connected toregulate the ionizing means and accordingly the magnitude of theelectron current in the collector whereby to maintain the ionization ata predetermined rate. means for producing a control impulse equal tothat of ionization at the predetermined rate and means for renderingsaid regulating means momentarily responsive to said impulse producingmeans to determine whether or not ionization is taking place at thepredetermined rate by observing whether or not a change occurs in themagnitude of the electron current.

7. In an ionization pressure gauge comprising ionizing means immersed ina gas the pressure of which is to be measured, in combination, meanscontrolling the ionizing means to maintain the ionization at apredetermined rate comprising means for deriving a voltage proportionalto the rate of ionization, a voltage source having a value equal to thevoltage derived from said predetermined rate of ionization and means forsubstituting the separate voltage source for the derived voltagein the.controlling means to check the accuracy of the regulation.

S'I'EPHEN M. MACNEILLE.

REFERENCES CITED The following references are of record in the I file ofthis patent:

UNITED STATES PATENTS Graw-Hill, 1917, pages 155-157.

